Their are various B.S and ATSM standards relating to bulk density, see The BMHB Glossary ot terms in Powder and Bulk Technology, but they tend to ignore time and void pressure effects. A useful introduction to the packing of particulate structures is given in W.Grey's book - 'The packing of Particles'. This is one of a series of publications edited by J.C.Williams in powder technology.

In practice, unless the particles have surface adhesion charactistics by virtue of their chemistry, a bed of particles that are greater than 100 microns will settle fairly readily to a stable condition, yet re-orient their contact points under vibration much more than by the application of normal loads. This is more of a random pile structural situation than one involving the escape of gas from the voids, which bedevils a mechanistic approach to the compaction of fine powders. Normal 'tapped density' measurments utilise a repeated small vertical drop of the container, usually up to 50 times over a distance of around 25mm. This combines the effect of a suddenly applied load with enhanced gravity. Greater shocks would tend to introduce elastic recovery, or bouncing effects' that could re-dilate the bed.

It is best to let fine particulates beds settle for some time, to allow the void pressure to approach ambient, before applying the 'tapping' process. It is convenient to use standard one litre measuring cylinders for such tests, so than the volume can be read of the markings directly. Wall friction will reduce the impact disturbance, but be less significant than when measuring 'poured' density or when external loads are applied to measure 'load-compacted' density. For these values it is recommended to use containers with a depth to diameter ration of about one that has very smooth internal walls.

This apparently simple subject has many interesting aspects and a comprehensive study would be a useful addition to the technical literature. I would hope to see comparisons of different techniques with diferent materials that have their wall friction values given. Best wishes in your studies. ■